The invention relates generally to an ultrasonic apparatus employed in eye surgery and more particularly to a new and improved ultrasonic cone and method of construction for emulsification of cataract lenses.
In the field of cataract lens eye surgery, many advances have taken place in the recent past to more efficiently remove the cataract lens from the eye. The cataract is a clouding of the lens of the eye which, over time, reduces the amount of light permitted to pass through the lens of the eye. Degeneration of the lens results in loss of sight in the eye and thus, the cataract must be removed and if opaque, the lens must be removed in total.
During the current advances of cataract lens removal, a plurality of hand-held instrumentalities were developed to assist in removing the cataract lens from the eye. Many of the instruments developed included finely controlled edges employed for cutting, scraping or puncturing the cataract lens for removing the opaque section blocking the passage of light. A more recent advance employs the use of emulsification which incorporates a high frequency ultrasonic source within the hand-held instrument for converting the electrical energy delivered to the ultrasonic source to a high frequency mechanically vibrating tip or cone.
The tip or cone of the ultrasonic instrument included one of a plurality of edges which was surrounded by a cylindrical sleeve or jacket which carried a sterile saline solution for irrigating the operative site within the lens of the eye. By physically touching the cataract lens of the human eye with the high frequency vibrating ultrasonic hand-held instrument, the hardened opaque cataract lens was emulsified and reduced to a liquid state. As the eye was irrigated with the saline solution, the emulsified debris of the destroyed cataract lens was vacuumed away by a low pressure suction from inside the hollow center of the tip or cone of the hand-held ultrasonic instrument. The saline solution and the emulsified debris were then disposed of through a channel connected to the end of the ultrasonic cone.
The hand-held instrumentality must be in physical contact with the lens of the eye in order for the ultrasonic sound waves to emulsify the fragments of the cataract lens within the eye. Therefore, the patient is normally under local anesthetic.
Various ultrasonic hand-held instrumentalities employed to emulsify the cataract lens of the human eye by applying a tip or cone directly to the cataract lens have been known for a number of years, and by way of example, several forms of such devices can be found in U.S. Pat. Nos. 4,515,583; 4,609,368; and 4,589,415.
The ultrasonic driver unit of each of the prior hand-held instrumentalities vibrated at a very high frequency. By way of example, a typical vibration frequency was 40 kHz. However, frequency ranges of (40 to 60) kHz are now available. The higher frequency ranges provide for a more rapid emulsification of the cataract lens leading to a shorter operable time for the patient.
In general, the instruments have included an outer housing comprised of an ultrasonic aspirator having the ultrasonic driver unit housed therein. The driver unit is linked to an operable probe or ultrasonic cone which is employed for making physical contact with the cataract lens. The driver unit is connected to the cone by a plurality of linkage designed to transmit the mechanical vibrations of the driver unit to the ultrasonic cone.
Typically, the ultrasonic aspirator housing further includes a cooling system designed to carry away heat generated by the frictional force of the ultrasonic cone and the driver unit. Also, the aspirator includes a fluid duct system for delivering the irrigating saline solution to the annular sleeve that surrounds the ultrasonic cone. The proximal end of the ultrasonic cone is typically threaded and is received by a female threaded horn permanently connected to the linkage between the driver unit and the cone.
The cone typically has a flange located forward of the cone threads and a hub that is employed for threading the cone into the horn with a mechanical instrument, for example, such as a wrench. The hub typically measures a larger dimension than that of the shaft of the ultrasonic cone, and it is the reduction of the dimension from the hub to the cone shaft that has been the subject of a major problem in the past.
Under prolonged use of the hand-held ultrasonic instruments, erosion of the material located between the hub and the shaft of the ultrasonic cone occurs. This erosion of the ultrasonic cone occurs in the interface where the radius makes a transition between the hub and the shaft of the cone. The ultrasonic mechanical vibrations travel up and down the shaft of the cone and across the transition radius of curvature which blends the hub to the shaft of the cone. The mechanical vibrations will strike the quasi-vertical portion of the radius interface resulting in mechanical damage and erosion of the ultrasonic cone in the region of the transition radius. Further, for a particular ultrasonic cone having a fixed transition radius, ultrasonic vibrations at a higher frequency or vibrations accompanied by a greater horizontal stroke will result in increased erosion.
The ultrasonic cone is typically comprised material which is neutral and non-chemically reactive with the human body, by way of example, such as titanium. Thus, if a portion of the titanium cone was to erode into flakes and fall into the human eye, then the human body would not build up an antibody resistance to the titanium and cause an infection impairing vision as would occur with other foreign substances.
The titanium cone erodes because the ultrasonic device sends the high frequency vibrations up and down the cone shaft. It is well known that the ultrasonic frequency is inversely proportional to the angle of incidence of the transition radius. The ultrasonic frequency of vibration is in most cases the determining factor, and for a fixed angle of incidence of the transition radius, higher ultrasonic frequencies result in increased erosion. Therefore, in the (40 to 60) kHz range of ultrasonic vibrations, visible particles of matter will erode from the ultrasonic cone which is evidenced by microscopic photographs of the cone after prolonged use.
Generally, as the ultrasonic frequency increases, the erosion of the ultrasonic cone further increases. As an example, experiments were conducted with an ultrasonic instrument employing an angle of incidence of 75 degrees and frequencies in the range of (40 to 60) kHz. Under these conditions, erosion was extremely excessive, resulting in substantial reduced life of the ultrasonic cone.
Another condition that will result in the erosion of the ultrasonic cone along the radius of curvature is the stroke of the shaft of the cone. The stroke of the cone shaft is most easily visualized as the back-and-forth horizontal vibration along the length dimension of the cone shaft. In the past, the horizontal stroke of the cone shaft was in the range of two-to-three thousandths of an inch but later was increased to the range of four-to-five thousandths of an inch. Although the increased stroke length improved the efficiency and speed of the aspirator in emulsifying a cataract lens, the erosion of the cone along the transition radius further reduced the useful life of the cone. Future designs anticipate increasing the horizontal stroke of the cone to the range of ten thousandths of an inch. Thus, increasing the vibration frequency or stroke of the cone having a fixed angle of incidence results in increased erosion of the cone.
Several attempts in the past to solve these problems have met with limited success. Any mechanical scores, scratches, or tool lines on the surface of the transition radius was an obvious site for erosion to occur. The scores, scratches and tool lines were potential stress points or stress-risers. The transition radius was necessary to blend the hub into the cone or shaft of the instrument. It was necessary to machine the outer surface of the transition radius to provide a smooth surface for reducing the likelihood of fracture. A first attempt to solve the erosion problem was to polish the surface where the erosion occurs. Therefore, any locations on the transition radius which included scores, scratches, or tool lines were polished in an effort to limit the erosion. However, the erosion continued to occur at frequencies greater than forty kilohertz.
Additional attempts to eliminate the erosion problem occurred during the manufacturing process of the ultrasonic cone. The first was acid etching of the entire ultrasonic cone which would remove any loose particulate matter or residue from the previous machining process. The acid etching reduced the ultrasonic cone to bare virgin metal.
The next manufacturing process included electro-polishing which is the opposite of electroplating. The process of electropolishing removes material from the surface of the ultrasonic cone which included particulate matter and residue from the previous machining process. In each of the manufacturing processes of acid etching and electropolishing, erosion continued to occur in the transition radius at frequencies greater than 40 kHz.
Other methods to prevent the erosion have also been attempted, one of which included providing a tapered transition in the region between the hub and the shaft of the ultrasonic cone, as is described, by way of example, in U.S. Pat. No. 3,589,363. Because erosion always occurred between the hub and the shaft along the transition radius of the ultrasonic cone, removal of that portion of the radius subject to erosion during manufacturing would eliminate the problem. However, if the erosion area was physically removed, the resulting surface would be too small in diameter to accommodate an installation tool normally employed to install the cone.
Therefore, the design of the transition radius was modified by construction methods for blending the material along the transition radius from the hub to the shaft. An example of such a method included constructing a line at a predetermined angle with the centerline of the shaft of the ultrasonic cone. Then, a radius of curvature was randomly selected so that the curve was tangent to the constructed line and the horizontal. Next, a line was constructed from the center of the curve to be perpendicular to an empirically determined point of tangency on the curve. Although a transition radius could be designed in this manner, an erosion area continued to exist substantially shortening the useful life of the cone.
A further construction technique which failed to solve the problem called for constructing a line at a specific angle and tangent to a corner of the hub. Then, a pair of vertical lines were drawn orthogonal to the constructed line. The constructed line was then bisected between the points of intersection of the two vertical lines and a third vertical line was drawn orthogonal to the constructed line at the bisection point and parallel to the pair of vertical lines. A fourth line was drawn perpendicular to the intersection point of the second of the pair of vertical lines on the shaft. Then, the fourth line was extended upward to intersect the third vertical line for establishing the center of an arc of the transition radius of curvature. However, the resulting transition radius failed to eliminate the erosion areas located along the quasi-vertical surfaces of the ultrasonic cone.
During the operation of the hand-held ultrasonic instrument, if the angle of incidence from the hub to the shaft of the ultrasonic cone is very large (for example, such as 75 degrees), the mechanical vibrations against the saline solution (cavitation) caused by increasing the frequency of vibration and the length of the stroke result in the formation of large bubbles along the transition radius. The large bubbles travel down the shaft of the ultrasonic cone and enter the eye at the site of the emulsification process impeding the view of the surgeon. To counteract this problem, a tube must be placed at the site of the emulsification process for vacuuming the large bubbles away from the surgical site.
Hence, those concerned with the development and use of hand-held ultrasonic instruments for the removal of cataract lens have long recognized the need for improved ultrasonic cones which reduce the erosion of the metal of the cone along the transition radius and also which reduce the cavitation and creation of bubbles along the transition radius to extend the life of the instrument and to reduce the visual impediments to the surgeon, respectively. The present invention fulfills all of these needs.